Raw or mixed hydrocarbon gas consists primarily of methane (CH4) but also includes heavier gaseous hydrocarbons such as ethane (C2H6), propane (C3H8), butanes (C4H10), pentanes (C5H12), higher molecular weight hydrocarbons (C6+), and other hydrocarbon species and non-hydrocarbons associated with the raw gas source.
The relative amount of the heavier gaseous hydrocarbons, or richness, can be expressed in terms of gallons per mcf (thousand cubic feet), abbreviated as GPM. In this embodiment, GPM includes all hydrocarbon components heavier than methane and represents the total volume, in liquid gallons, contained in one thousand cubic feet of a particular gas at standard conditions.
The raw gas must be purified to produce a gas product that meets the quality standards specified by a particular gas transmission pipeline (“sales gas”). Typically, one of the objectives of gas processing is to remove liquefiable hydrocarbons commonly referred to as hydrocarbon gas liquids (HGL) comprised of propane, butanes, pentanes and higher molecular weight C6+ hydrocarbons to meet the pipeline gas quality specification desired. A second objective is often then to remove further HGL (deeper cut, e.g., incremental C3+ and perhaps even ethane, C2) for economic gain. If the raw gas stream contains objectionable quantities of non-hydrocarbon compounds such as sulphur compounds or carbon dioxide, these are typically removed by pre-treatment processes not shown or discussed here. Similarly, excess water vapor in the raw gas stream is removed through dehydration.
A conventional approach to removing the HGL is to use the straight refrigeration process as described in the Gas Processors Suppliers Association Engineering Data Book, Chapter 16, 13th Edition. While there are several configurations known in the art for the straight refrigeration process, FIG. 1 (Prior Art) depicts a configuration that is commonly used in the industry and is hereinafter referred to as “conventional refrigeration process”. In the conventional refrigeration process shown in FIG. 1, a rich gas feed (stream 110) is combined with recycle gas (stream 127) to produce stream 111. Stream 111 is then separated into two steams, stream 112 and stream 114, where stream 114 enters a gas-to-liquid heat exchanger 101 and stream 112 enters a gas-to-gas heat exchanger 102. The heat exchangers reduce the temperature of the gas streams 112 and 114, which streams exit as cooled gas stream 113 and cooled gas stream 115, respectively. Streams 113 and 115 are then combined and combined cool gas stream (stream 116) is further chilled in a second heat exchanger (gas chiller 103), which uses mechanical refrigeration, typically using propane as the refrigerant, but could use any refrigerant type, and could include using Joule-Thompson expansion cooling. The desired temperature of the cold gas 117 produced is dependent on the raw gas composition, the pressure of the raw gas stream, the gas quality specification desired, and the economics of recovering additional liquids.
The cold gas 117 is sent to a cold gas separator (cold separator 105), which is also referred to in the literature as a low temperature separator or LTS, where the condensed liquids 122 are separated from gas. The residual gas stream 118 from the cold separator 105 is returned to the gas-to-gas heat exchanger 102, and is warmed by the incoming raw gas stream 112. The warmed residual gas 119 is dry relative to the rich gas feed stream 110 and is often intended to be conveyed to the gas transmission pipeline for sale (dry sales gas 120).
Liquids 122 from the cold separator 105 are warmed in the gas-to-liquid exchanger 101 and the warmed liquids 124 are then expanded through adjustable valve 106 which functions to hold a constant liquid level in the cold separator 105 and reduce the liquid stream pressure (stream 125) before stream 125 enters the fractionation column or tower 108. Heat exchangers 101 and 102 reduce the energy requirement in gas chiller 103, by utilizing the energy already expended to chill the cold gas stream and transferring energy from warm to cold streams.
The fractionation tower 108 comprises a reboiler 156, which provides heat and generates vapors to drive the distillation or fractionation process. The fractionation tower 108 distills the co-absorbed light components (primarily C1, C2, and sometimes C3, and C4 hydrocarbons) from the liquid stream to meet the HGL quality specifications of a liquids transporter and downstream refinery or fractionation facility. The fractionation tower may further comprises a reflux condenser (not shown) to improve separation of the light hydrocarbon components from the heavier ends liquid stream.
However, the composition of the HGL from the conventional refrigeration process is somewhat inflexible as discussed later,
The overhead gas stream 126 from the fractionation tower 108 is then compressed in overhead compressor 109 to produce recycle gas 127. Recycle gas 127 is recycled by combining with the rich gas feed 110 and reprocessed. The bottom liquids product 130 contains the HGL extracted from the gas stream. In one alternative, the overhead compressor gas 109 can be added as gas 128 to residual gas 119 to produce dry sales gas 120.
In the conventional refrigeration process, the “mid-components”, which are defined herein as C3 and C4 hydrocarbons, are extracted from the rich gas feed as liquid product and are therefore found in the HGL product. However, there may be times when market demand and product pricing does not economically support the extraction of the propane and butane from the raw gas, and, therefore, rejection of these constituents from the HGL product to the gas product is desired. In other words, there is more value for these mid-components to remain in the gas stream rather than be recovered as HGL product, provided gas transmission pipeline specifications are met.
There may also be times where the extraction of the heavier hydrocarbons is desired close to the source of the raw gas, and the extraction of the mid-components is desired at an alternate downstream gas processing plant, generally distant from the source and which feedstock often comprises an aggregation of several residue gas streams, to achieve economies of scale and close proximity to consumer markets.
To maintain these mid-components in the residue gas steam in a conventional refrigeration process, conventional practice has been to: adjust the temperature in gas chiller 103 to a higher temperature so as to reduce the amount of these constituents condensed and separated in cold separator 105 as liquid stream 122; and/or adjust the operating conditions in the fractionation tower 108 and reboiler 156 in order to reject the co-absorbed mid components into stream 126 and recycle stream 127 back into the rich gas feed stream 110; and in very rich raw gas feed streams, conventional practice is to re-direct the fractionation overhead gas stream 127 from combining with the raw gas feed stream 110, to combining with the residue gas stream 119, via stream 128, to form dry Sales Gas 120.
However, the above practices to maintain mid-components in the dry sales gas stream 120 can result in low recoveries of the constituents that are desirable in the HGL product stream 130. In other words, economic value is lost because a portion of the heavier hydrocarbons such as C6+, pentanes, and sometimes butanes remain in the dry sales gas stream 120. Therefore, there is a need in the industry for a process and apparatus that is capable of customizing the amount of propane and butane retained in a sales gas product from a raw gas stream without compromising the recovery of valuable heavy hydrocarbons such as C5 and C6+ components (HGL products) from the raw gas stream.